greybus: abandon incoming requests for now

Change the operation "receive workqueue" to be just the operation
"workqueue".  All it does is complete an operation in non-atomic
context.  This is all that's required for an outgoing request.

Similarly, ignore any notion that a response will only exist
for outgoing requests in gb_operation_cancel().

I'm doing this in the interest of getting the outgoing request path
verified without the encumbrance of any preconceptions about how
incoming requests need to work.  When I finally turn my full
attenion to incoming requests I'll adapt the code as needed.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>